Research On Hydrogen Induced Cracking And Soil Stress Corrosion Cracking Of X120 Pipeline Steel

As the growing demand of energy, the high grade pipeline steel has been considerable development in the world as the main transport way. As the material of choice for large pipeline projects because of the advantages they provide such as low price-to-yield ratio and low carbon content which impact material, welding and installation costs. So use the high grade pipeline steel for transport will be the inevitable. Currently, the X70 and X80 pipeline has been large-scale used in engineering, the X100 and X120pipline has been carried out extensive research and trial work at home and abroad.The corrosion damage is the greatest harm for the pipeline steel. The pipeline service environment system can be divided into tube ambient and external environment. The tube ambient environment refers to the tube wall inner situation. Always the ambient environment contains H2S. It could induce two kinds of failure: One is Hydrogen induced cracking, the other is sulfide stress corrosion cracking. The tube external environment refers to the all kinds of soil environment and geological environment. The corrosion and stress corrosion cracking will be occurred when the coating peeling. A lot of research about the failure which mentioned above has been done by previous scholars, but it almost focus on the steel that below the lever of X70. This paper expand a study on the hydrogen induced cracking of X120 steel in H₂S environment and soil stress corrosion cracking of it in acid soil environment. The main results show that: The cracks propagate direction with the same of rolling. The HIC almost found at the center of steel cross-section. The type of crack propagation was intergranular along the prior austentic grain boundaries, the lath bainite microstructure had bad hydrogen induced cracking resistance. The lath bainite microstructure had good hydrogen hydrogen induced cracking resistance. The ability of microstructures and inclusions to trap hydrogen was explained in terms of the apparent diffusivity (Dapp), permeability (J_∞L), and solubility of hydrogen in the steels tested (capp). The lower values of Dapp, J_∞L and a higher value of capp suggest that, more hydrogen can be entrapped in the steel and the consequently the steels are more susceptible to HIC. The corrosion calculated by two different methods was close. The based internal rust layer which composited with FeO(OH)and Fe3O4 can be better to impede the corrosion process. With increasing time, the corrosion process from concentration polarization gradually transform to activation polarization.When X120 steel test at OCP in acid solution, the cracking was the type of anodic dissolution, and the susceptibility of X120 was not very extremely. When the cathodic potential is -0.65VSCE the stress corrosion cracking susceptibility of X120 steel is low. However, when the cathodic potential is below -1.0 VSCE, the stress corrosion cracking susceptibility was very high which hydrogen assisted cracking becomes dominated process.